U.S. Pat. No. 2,272,342, granted Feb. 10, 1942 to J. S. Hyde, describes a glass preparation technique based on the so-called flame hydrolysis process which employs vapor phase oxidation of high purity vapors to form finely divided particles of glass referred to as "soot". This soot may be collected in particulate form, or may be deposited on a mandrel or other support as a preform. The preform may be consolidated in position, or by a separate heat treatment.
Subsequent patents including U.S. Pat. Nos. 2,326,059 granted Aug. 3, 1943 to M. E. Nordberg and 2,239,551 granted Apr. 22, 1941 to R. H. Dalton et al. describe methods of producing glasses by flame hydrolysis wherein the glass is composed of an oxide mixture, in particular a fused silica-type glass incorporating small amounts of one or more additional oxides such as titania or alumina. In general, these methods involve forming a mixture of volatile compounds in the desired proportions in a stream of gas fed to a combustion burner. The mixture of vapors is then oxidized to deposit a glass body or preform composed of a corresponding oxide mixture.
The processes described in the aforementioned Hyde, Dalton and Nordberg patents are capable of preparing very high purity glasses and therefore have formed the basis for recent improved methods of forming low loss glass optical waveguide filaments. The flame hydrolysis technique has been employed to prepare single mode waveguides and multimode waveguides of both the step-index and graded-index type. Various methods employing the flame hydrolysis as well as other techniques for forming such filaments are taught in U.S. Pat. Nos. 3,737,292; 3,823,995 and 3,884,550. In accordance with the teachings of these three last mentioned patents, a coating of glass soot is applied radially to a substantially cylindrical mandrel or starting member by means of a flame hydrolysis burner. Fuel gas and oxygen or air are supplied to the burner, and this mixture is burned to produce a flame. The vapor of a source material is introduced into the flame and the resultant reaction forms a glass soot which is directed toward the mandrel. The mandrel may be rotated and translated with respect to the flame to form uniform glass soot coatings thereon. Since the resultant filament consists of a glass core surrounded by a layer of cladding glass having a refractive index lower than that of the core, the refractive index of the first deposited layers must be higher than that of the finally deposited layers. This is usually accomplished by employing the flame hydrolysis process to form the core of a base glass to which there has been added a dopant to increase the refractive index thereof. The cladding layer can then consist of the base glass alone or combined with a smaller amount of the same dopant material or combined with another dopant material which provides the desired lower refractive index.
Another method of utilizing the flame hydrolysis process to form optical waveguide filaments is taught in U.S. Pat. No. 4,062,655. In accordance with the teachings of that patent the burners are located longitudinally of the endface of the starting member so that the glass soot produced thereby deposits on the endface and builds up longitudinally. Since the starting member is disposed at the end of the resultant preform, there is no aperture therethrough.
Mixed oxide glasses can be produced by these earlier methods, but it is frequently difficult to avoid condensation during delivery of certain vapors at elevated temperatures to a combustion flame. It may also be difficult to control relative rates of vaporization of some reactants, and hence proportions of certain oxides in the final product. Two improved methods of introducing an oxide additive into a glass produced by flame hydrolysis are disclosed in my U.S. Pat. Nos. 3,859,073 and 3,864,113. Both of these methods require, as an initial step, that particles of the primary glass former be produced by flame hydrolysis and deposited to form a porous body. In accordance with the teachings of U.S. Pat. No. 3,859,073 the porous body or preform is impregnated, in part at least, with a dopant which may be dissolved or suspended in a vehicle which must be removed by a method such as heating, air drying or the like. The method of U.S. Pat. No. 3,864,113 requires that the preform be impregnated with a vaporized dopant which condenses within the pores on cooling. In accordance with the teachings of both of these patents, the preform is then thermally consolidated with the dopant dispersed therein. After the pores of the preform have been impregnated by either of the two aforementioned techniques, the preform is subjected to a sintering or consolidating heat treatment to form a dense glass body free from particle boundaries.
U.S. Pat. No. 3,938,974 issued to P. B. Macedo et al. related to a method of adding a dopant to another type of porous glass preform. A glass which is separable into at least two phases, one of which is soluble, is heated to cause phase separation. The soluble phase is leached out to form a porous glass preform, the interconnected pores of which are thereafter stuffed with a dopant. The preform is then dried, thereafter consolidated to close the pores thereof and then drawn into a fiber.
In my two U.S. Pat. Nos. 3,859,073 and 3,864,113 and in the Macedo patent, there is initially formed a porous preform having a substantially constant refractive index throughout, and that preform is subjected to a dopant-containing vapor or liquid and thereafter cooled or dried to form a composite body consisting of a porous matrix having a dopant material dispersed throughout. Subsequently, the doped porous preform is thermally consolidated to form a dense glass body. It is thus seen that these prior art techniques require a number of sequentially performed steps to form a doped, dense glass body from a porous glass preform.